If you have never owned a 360 before, and you want to mod one to play backups, here's some advice:
The newer it is, the harder it is to get the DVD key. The sweet spot for easy modding is a manufacturing date of about April 2008 to August 2009. The easiest and safest drive is a BenQ.
to learn what to look for and where on the console. Like I said, you want a BenQ, NOT a Lite-On.
Don't get a brand new one, and DON'T get a Slim (they're not moddable yet). Go to a place like GameStop, and tell the clerk what you're looking for, but don't mention modding. Check the manufacture date on the back. Have them take off the front plate so you can look at the hole under the tray (to identify the drive type), and also check the warrenty sticker. If the sticker is damaged, make sure you have the clerk note that on the receipt. 30 day return policies are sweet!
All the Xecuter 360 Blaster CK3 does is allow you to read/flash your drive without having to open your case every time. You'll have to open it to install the Blaster CK3, but that's it.
As far as flashing is concerned, go to http://www.jungleflasher.net/downloads.html
Read the tutorial for how to set up your PC, and how to use Jungleflasher with your specific drive. Follow the tutorial, and you should have no problems. Enjoy!